Since a metal-belt type or metal-chain type continuously variable transmission allows selective utilization of high combustion efficiency ranges of an engine, it has been attracting attention as a transmission of excellent fuel saving capability. In particular, an increasing number of car models are equipped with a metal-belt continuously variable transmission in recent years. The metal-belt or metal-chain continuously variable transmission is so configured that the torque is transmitted via the friction between the metal belt or chain and the metal pulley, and the speed is changed by varying the pulley radius ratio. Therefore, lubricating oil used for the metal-belt or metal-chain continuously variable transmission is required to provide a high metal-to-metal friction coefficient in order to improve the torque capacity.
A method of adding zinc dialkyldithiophosphate has been proposed for increasing the metal-to-metal friction coefficient (see Non-patent Document 1). However, zinc dialkyldithiophosphate gets worn out through use, causing the problem of decreased metal-to-metal friction coefficient. Moreover, some of the belt-type continuously variable transmissions nowadays are combined with a wet clutch, which is problematic because in such a situation the deterioration products of the zinc dialkyldithiophosphate tend to clog the clutch plate and thus impair its function.
Therefore, a variety of continuously variable transmission fluids that do not contain zinc dialkyldithiophosphate have been proposed, such as a continuously variable transmission oil comprising calcium salicylate, a phosphorus antiwear agent, a friction modifier and a dispersion-type viscosity index improver (Patent Document 1), a continuously variable transmission fluid comprising an ashless polyisobutenyl succinimide dispersant, an organic phosphite, a calcium overbased phenate detergent, a friction modifier containing a succinimide and an ethoxylated amine, and a primary amide of a long-chain carboxylic acid (Patent Document 2), a continuously variable transmission oil comprising a specific mineral lubricating oil base oil, a phosphorous compound and a boron-modified succinimide as ashless dispersant (Patent Document 3), a continuously variable transmission oil comprising a specific phosphorus compound and a boron-modified succinimide as ashless dispersant (Patent Document 4), and a continuously variable transmission oil comprising a boron-containing succinimide ashless dispersant (Patent Document 5).
However, the above transmission oils do not provide a satisfactory friction coefficient for transmitting the high output of high-power engines, and further improvements have thus been desired.
Meanwhile, a use of a lubricant or a functional oil comprising a combination of a specific hydrocarbon-soluble aryl phosphate and a specific hydrocarbon-soluble aryl polyphosphate as antiwear agent has been proposed (Patent Document 6). However, Patent Document 6 does not disclose that the aryl polyphosphate may be used as an additive for the continuously variable transmission oil or that it may increase the metal-to-metal friction coefficient.